1. Field of the Invention:
The present invention relates to a field-effect transistor having a double-heterojunction structure, and more particularly to a field-effect transistor with improved transconductance.
2. Description of the Prior Art:
One known field-effect transistor having a double-heterojunction structure includes a channel layer of indium gallium arsenide (InGaAs) having a thickness ranging from 100 to 200 angstroms, and a pair of wide bandgap layers (electron supply layers) of aluminum gallium arsenide (AlGaAs) which is disposed one on each side of the channel layer. Such a conventional field-effect transistor is disclosed in Japanese laid-open patent publications Nos. 1-128473 and 1-143271, for example.
FIG. 1 of the accompanying drawings shows drain voltage vs. drain current characteristics of such a conventional field-effect transistor at various gate voltages used as a parameter. A study of FIG. 1 indicates that the transconductance g.sub.m of the conventional field-effect transistor is not uniform.
To solve the above problem, it has been proposed to make the concentration of the dopant or impurity introduced into one of the wide bandgap layers greater than the concentration of the dopant introduced into the other wide bandgap layer, or to make the bandgap of one of the wide bandgap layers greater than the bandgap of the other wide bandgap layer, for thereby uniformizing the transconductance g.sub.m.
However, the proposed solutions, which are based on individual adjustments of the dopant concentrations and the bandgaps, are unable to both increase and uniformize the transconductance g.sub.m because an attempt to uniformize the transconductance g.sub.m results in a reduction in the transconductance g.sub.m.